Thank you for your replies and advice.
I have noticed their tendency to shoot up and they do have large spaces between the nodes. This is the reason I want to cut them, to try to reduce the diameter : height ratio. I will cut above the first node on one of them and see how it behaves. It's late autumn here now, temperatures will drop below zero in a couple of weeks so I think I'll let them sleep and do this in spring.
Unfortunately I will not be able to plant them elsewhere than in a pot. The soil is quite woody and was sold as soil for indoor plants. I will have to do something about the drainage I guess.
They are my test plants, but would hate to kill them. I don't mind waiting (how long is that?) if a nice little bush will grow eventually.
I encourage you to keep growing these trees. You can get good experience from them. But you should really plan on creating trees between 3 feet and 5 feet tall if you want a convincing bonsai with this species. I and the others say this because we have tried it ourselves. We've been there, so to speak.
If you keep them in small pots, it may take 30 or 40 years to get any size on them. You will be able to enjoy them, and enjoy the 'zen' of the experience that goes into taking care of them. Both are worth while pay offs, but they won't be show worthy in less than several decades.
I would pot them up into 5 gallon nursery containers if you don't want to put them in the ground. Repot every couple years and prune roots with an eye toward developing a nice, well branched root base. Relax and enjoy the meditative experience of caring for the trees. Doing this may give you nice results in a MUCH shorter time span, ten years or so and you will have trees that you can actually do some real styling on. Until them you are basically building the trunks and roots bases. During this time read, and attend bonsai shows and learn more.
For smaller bonsai, I really recommend using one of the species recommended more often for bonsai. Japanese Maples are excellent, as are trident maples. Amur maple is great for medium size trees and larger trees. All have shorter internodes and smaller leaves. American red maple is curious, has nice leaves that reduce somewhat in size, but the leaf petiole (the stem that attaches the leaf to the tree) the leaf petiole does not reduce in proportion with other elements of the tree, which gives an odd look. A nice medium size tree, with leaves that dangle out on long stems. But the American red maple (Acer rubrum) has nice fall color and a nice look as a bare winter tree. Worth a try if you want to use a local native.
Hope the thought helps. You need to start a lot of pots of trees, so you have enough to do while waiting for your seedlings to grow. The solution to being impatient is to get more trees!
